Isolated switching mode power supply and the method thereof

ABSTRACT

An isolated switching mode power supply, having: an input terminal; an output terminal; a transformer having a primary winding and a secondary winding; a primary power switch coupled to the primary winding; a secondary power switch coupled between the secondary winding and the output terminal of the power supply; a secondary controller configured to generate a frequency modulation signal based on the output voltage and the first feedback signal; a coupled device configured to provide a frequency control signal based on the output voltage and the frequency modulation signal; and a primary controller configured to provide a switching signal to control the primary power switch based on the current sense signal and the frequency control signal.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and the benefit of Chinese PatentApplication No. 201210139417.7, filed May 8, 2012, which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to switching mode powersupplies, and more particularly but not exclusively to isolatedswitching mode power supplies and the method thereof.

BACKGROUND

Primary side control is widely applied in conventional isolatedswitching mode power supplies. FIG. 1 schematically shows a prior artisolated switching mode power supply 10. As shown in FIG. 1, atransformer T1 comprising a primary winding Lp, a secondary winding Lsand a third winding Lt is applied as energy storage element in the powersupply 10. A power switch integrated in primary chip IC1 is coupled tothe primary winding Lp via terminal “D”. The power switch is controlledto be ON/OFF so as to store energy in the primary winding Lp or totransfer energy from the primary winding Lp to the secondary winding Ls.Then the secondary winding Ls transfers the energy to the outputcapacitor Co to generate an output voltage Vo. In FIG. 1, the powerswitch and a control circuit controlling the power switch are integratedin chip IC1. The chip IC1 has a feedback terminal FB configured toreceive the output voltage Vo via an OPTO-coupler D0. The controlcircuit controls the power switch based on the output voltage Vo and acurrent flowing through the primary winding Lp.

As can be seen from the above description, in order to maintain theproper work of the power supply 10, the opto-coupler D0 should keepworking during the normal operation. Persons of ordinary skill in theart should know that the power consumption of the opto-coupler D0 and aresistor R3 coupled to the opto-coupler D0 are constant either underlight load or heavy load. Thus the power consumption of the opto-couplerD0 and the resistor R3 accounts for a large proportion of the wholepower consumption of the power supply 10 when the load is light,especially when there is no load.

SUMMARY

It is an object of the present invention to provide an improved isolatedswitching mode power supply and the method thereof to solve the aboveproblems.

In accomplishing the above and other objects, there has been provided,in accordance with an embodiment of the present invention, an isolatedpower supply comprising: an input terminal configured to receive aninput voltage; an output terminal configured to provide an outputvoltage; a transformer having a primary winding and a secondary windingrespectively having a first terminal and a second terminal, the firstterminal of the primary winding being coupled to the input terminal toreceive the input voltage; a primary power switch having a firstterminal coupled to the second terminal of the primary winding, a secondterminal coupled to a primary ground node and a control terminal; asecondary power switch coupled between the first terminal of thesecondary winding and the output terminal of the power supply; asecondary controller having a power terminal configured to receive theoutput voltage, a first feedback terminal configured to receive a firstfeedback signal indicative of the output voltage and a coupling controlterminal configured to generate a frequency modulation signal based onthe output voltage and the first feedback signal; a coupled devicehaving an input side coupled between the output terminal of the powersupply and the coupling control terminal to receive the output voltageand the frequency modulation signal, and an output side configured toprovide a frequency control signal based on the output voltage and thefrequency modulation signal; and a primary controller having a currentsense terminal configured to receive a current sense signal indicativeof a current flowing through the primary winding, a frequency controlterminal coupled to the output side of the coupled device to receive thefrequency control signal, and an output terminal configured to provide aswitching signal to the control terminal of the primary power switchbased on the current sense signal and the frequency control signal.

Furthermore, there has been provided, in accordance with an embodimentof the present invention, a method of controlling an isolated switchingmode power supply, the isolated switching mode power supply comprising atransformer, a primary power switch and a secondary power switch,wherein the transformer has a primary winding, and a secondary winding,and wherein the primary power switch is coupled to the primary windingand the secondary power switch is coupled to the secondary winding, themethod comprising: receiving an input voltage via the primary winding ofthe transformer; turning ON and OFF the primary power switch to transferenergy stored in the primary winding to the secondary winding, and toprovide an output voltage to the load; generating a first feedbacksignal indicative of the output voltage; generating a frequencymodulation signal based on the first feedback signal and a sawtoothsignal; generating a frequency control signal based on the frequencymodulation signal; generating a current sense signal indicative of acurrent flowing through the primary winding of the transformer;generating a current limit signal based on the current sense signal andthe peak current signal; and generating a switching signal to controlthe primary power switch based on the current limit signal and thefrequency control signal.

In addition, there has been provided, in accordance with an embodimentof the present invention, a method of controlling an isolated switchingmode power supply, the isolated switching mode power supply comprising atransformer, a primary power switch and a secondary power switch,wherein the transformer has a primary winding and a secondary winding,and wherein the primary power switch is coupled to the primary windingand the secondary power switch is coupled to the secondary winding,comprising: receiving an input voltage via the primary winding of thetransformer; turning ON and OFF the primary power switch to transferenergy stored in the primary winding to the secondary winding, and toprovide an output voltage to the load; generating a first feedbacksignal indicative of the output voltage; generating a frequencymodulation signal based on the first feedback signal and a sawtoothsignal; generating a frequency control signal based on the frequencymodulation signal; generating a current sense signal indicative of acurrent flowing through the primary winding of the transformer;generating a current limit signal based on the current sense signal andthe peak current signal; and generating a switching to control theprimary power switch based on the current limit signal and the frequencycontrol signal.

The presented isolated switching mode power supply and the methodthereof reduce the power consumption to improve the efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a prior art isolated switching mode powersupply 10.

FIG. 2 schematically shows an isolated switching mode power supply 20 inaccordance with an embodiment of the present invention.

FIG. 3 shows the waveforms of the signals of the isolated switching modepower supply 20 in FIG. 2.

FIG. 4 schematically shows a primary controller 204 in accordance withan embodiment of the present invention.

FIG. 5 shows a flow chart 50 of a method of controlling an isolatedswitching mode power supply in accordance with an embodiment of thepresent invention.

FIG. 6 shows a flow chart 60 of a method of controlling an isolatedswitching mode power supply.

The use of the same reference label in different drawings indicates sameor like components.

DETAILED DESCRIPTION

In the present invention, numerous specific details are provided, suchas examples of circuits, components, and methods, to provide a thoroughunderstanding of embodiments of the invention. Persons of ordinary skillin the art will recognize, however, that the invention can be practicedwithout one or more of the specific details, and could be adopted inmany applications besides the phase-shift dimming circuits, for example,the invention could also be applied in interleaving circuits. In otherinstances, well-known details are not shown or described to avoidobscuring aspects of the invention.

FIG. 2 schematically shows an isolated switching mode power supply 20 inaccordance with an embodiment of the present invention. The isolatedswitching mode power supply 20 comprises: an input terminal configuredto receive an input voltage Vin; an output terminal configured toprovide an output voltage Vo; a transformer T1 having a primary windingLp, a secondary winding Ls and a third winding Lt respectively having afirst terminal and a second terminal, the first terminal of the primarywinding Lp being coupled to the input terminal to receive the inputvoltage Vin; a primary power switch M1 having a first terminal coupledto the second terminal of the primary winding Lp, a second terminalcoupled to a primary ground node PGND and a control terminal; asecondary power switch D1 coupled between the first terminal of thesecondary winding Ls and the output terminal of the isolated switchingmode power supply 20; a secondary controller 202 having a power terminalVcc configured to receive the output voltage Vo, a first feedbackterminal FB1 configured to receive a first feedback signal Vfb1indicative of the output voltage Vo, and a coupling control terminal OPconfigured to generate a frequency modulation signal based on the outputvoltage Vo and the first feedback signal Vfb1; a coupled device havingan input side 101-1 coupled between the output terminal of the isolatedswitching mode power supply 20 and the coupling control terminal OP toreceive the output voltage Vo and the frequency modulation signal, andan output side 101-2 configured to provide a frequency control signalCon based on the output voltage Vo and the frequency modulation signal;and a primary controller 201 having a current sense terminal CSconfigured to receive a current sense signal Vcs indicative of a currentflowing through the primary winding Lp, a frequency control terminal FREcoupled to the output side 101-2 of the coupled device to receive thefrequency control signal Con and an output terminal Drv configured toprovide a switching signal Gate to the control terminal of the primarypower switch M1 based on the current sense signal Vcs and the frequencycontrol signal Con.

In the example of FIG. 2, the coupled device comprises an opto-coupler.The input side 101-1 of the opto-coupler comprises a light emittingdiode, and the output side 101-2 of the opto-coupler comprises anoptical transistor. The light emitting diode has an anode and a cathode,wherein the anode is coupled to the output terminal of the isolatedswitching mode power supply 20 via a resistor R2 to receive the outputvoltage Vo, and the cathode is coupled to the coupling control terminalOP of the secondary controller 202 to receive the frequency modulationsignal. The optical transistor has a first terminal coupled to theprimary ground node PGNG, a control terminal configured to sense thelight of the light emitting diode, and a second terminal configured toprovide a frequency control signal Con based on the light of the lightemitting diode. Persons of ordinary skill in the art should know thatthe light of the light emitting diode indicates the output voltage Voand the frequency modulation signal. Thus the frequency modulationsignal is transmitted to the optical transistor. As a result, thefrequency control signal Con has similar waveform with the frequencymodulation signal. Persons of ordinary skill in the art should know thatthe resistor R2 is configured to limit the current flowing through thelight emitting diode. In some embodiments, the resistor R2 may beomitted. The light emitting diode and the resistor R2 may be coupled inother ways, for example, the light emitting diode and the resistor R2may switch their positions. Persons of ordinary skill in the art shouldknow that the opto-coupler and the resistor R2 are configured to convertthe frequency modulation signal at the secondary side to the frequencycontrol signal at the primary side. Any suitable circuit performing theabove function may be used without detracting from the merits of thepresent invention. The operation of the opto-coupler is known to personsof ordinary skill in the art and is not described here for brevity.

In one embodiment, the secondary controller 202 comprises: an erroramplifier 102 having a first input terminal (non-inverting inputterminal) configured to receive the first feedback signal Vfb1indicative of the output voltage Vo, a second input terminal (invertinginput terminal) configured to receive a first reference signal Vref1 andan output terminal configured to provide an error signal Vc based on thefirst feedback signal Vfb1 and the first reference signal Vref1; anerror comparator 103 having a first input terminal (inverting inputterminal) coupled to the output terminal of the error amplifier 102 toreceive the error signal Vc, a second input terminal (non-invertinginput terminal) configured to receive a sawtooth signal Vsaw and anoutput terminal configured to provide a first comparison signal based onthe error signal Vc and the sawtooth signal Vsaw; and a first switch M2having a first terminal coupled to the coupling control terminal OP ofthe secondary controller 202, a second terminal coupled to a secondaryground node SGND and a control terminal coupled to the output terminalof the error comparator 103 to receive the first comparison signal,wherein based on the first comparison signal, the first switch M2 isturned ON and OFF to generate the frequency modulation signal at thecoupling control terminal OP.

In one embodiment, the secondary controller 202 further comprises asawtooth generator 104 coupled to the connection node of the secondarywinding Ls and the secondary power switch D1 to receive a synchronoussignal and to provide the sawtooth signal Vsaw. The operation of thesawtooth generator 104 is: when the secondary power switch D1 is tunedON, the synchronous signal is logical high and the sawtooth signal Vsawincreases; when the sawtooth signal Vsaw reaches the error signal Vc,the sawtooth signal Vsaw decreases to be logical low. The sawtoothsignal Vsaw increases again when the secondary power switch D1 is turnedON in the next switching cycle.

In one embodiment, the synchronous signal is omitted. The logical lowtime of the sawtooth signal Vsaw is preset to a constant time period t.That is to say, the sawtooth signal Vsaw increases after a constant timeperiod t, and becomes logical low when it reaches the error signal Vc.And after a fixed time period t, the sawtooth signal Vsaw increasesagain. The operation repeats so that the sawtooth signal has a waveformas shown in FIG. 3. The constant time period t may be adjusted indifference systems.

In one embodiment, the primary controller 201 comprises: a current limitcomparator 107 having a first input terminal configured to receive thecurrent sense signal Vcs, a second input terminal configured to receivea peak current signal Vlim, and an output terminal configured to providea current limit signal Vp based on the current sense signal Vcs and thepeak current signal Vlim; and a logic circuit 108 having a first inputterminal coupled to the output side 101-2 of the coupled device toreceive the frequency control signal Con, a second input terminalcoupled to the output terminal of the current limit comparator 107 toreceive the current limit signal Vp, and an output terminal configuredto provide a logic control signal 111 based on the frequency controlsignal Con and the current limit signal Vp.

In one embodiment, the logic control signal 111 provided by the logiccircuit 108 is applied as the switching signal Gate to control theprimary power switch M1.

In the example of FIG. 2, the frequency control signal Con is anactive-low signal. The logic circuit 108 comprises: a first inverter 109having an input terminal coupled to the output side 101-2 of the coupleddevice to receive the frequency control signal Con and an outputterminal configured to provide the inverted frequency control signal; afirst RS flip-flop 106 having a set terminal “S” coupled to the outputterminal of the first inverter 109 to receive the inverted frequencycontrol signal, a reset terminal “R” coupled to the output terminal ofthe current limit comparator 107 to receive the current limit signal Vpand an output terminal “Q” configured to provide the logic controlsignal 111 based on the inverted frequency control signal and thecurrent limit signal Vp.

In some embodiments, the frequency control signal Con is an active-highsignal. Thus the first inverter 109 may be omitted.

FIG. 3 shows the waveforms of the signals of the isolated switching modepower supply 20 in FIG. 1 and FIG. 2, wherein: lload represents the loadcurrent, i.e., a current flowing through the load resistor RL; GD1represents the ON and OFF of the secondary power switch D1, and whereinGD1 is high when the secondary power switch D1 is ON and is low when thesecondary power switch D1 is OFF; and Gate represents the switchingsignal which controls the primary power switch M1. In the example ofFIG. 2, the primary power switch M1 is turned ON when the switchingsignal Gate is logical high, and is turned OFF when the switching signalGate is logical low.

The operation of the isolated switching mode power supply 20 will bedescribed with reference to FIGS. 2 and 3. The output voltage Vo of theisolated switching mode power supply 20 maintains when the isolatedswitching mode power supply 20 works in steady state. As shown in FIG.3, the load changes from heavy to light at time t0. Then, the outputvoltage increases slightly, followed by the increase of the firstfeedback signal Vfb1. As a result, the error signal Vc increases. Thenthe time period of the sawtooth signal Vsaw increasing to reach theerror signal Vc is prolonged for the rising slope of the sawtooth signalVsaw is constant. Thus, the OFF time of the first switch M2 increases.In one embodiment, the frequency control signal Con is logical high whenthe first switch M2 is turned OFF, and is logical low when the firstswitch M2 is turned ON. So the logical high time of the frequencycontrol signal Con is prolonged when the load changes from heavy tolight. The frequency control signal Con is inverted to set the first RSflip-flop 106, so as to turn ON the primary power switch M1. As aresult, the OFF time of the primary power switch M1 increases. Thus, theenergy transferred to the load resistor RL is reduced and the loadcurrent decreases so as to adapt to the change of the load. At time t1,the load changes from light to heavy. At this time, the output voltagedecreases slightly, followed by the decrease of the first feedbacksignal Vfb1. As a result, the error signal Vc decreases. Based on thesimilar principle of when the load changes from heavy to light, the OFFtime of the first switch M2 decreases. So the logical high time of thefrequency control signal Con is shortened. As a result, the OFF time theprimary power switch M1 decreases. Thus, the energy transferred to theload resistor RL is increased and the load current increases so as toadapt to the change of the load.

In the switching mode power supply 20, the coupled device is idle whenthe first switch M2 is turned OFF. Persons of ordinary skill in the artshould know that the power consumption of the coupled device is almostzero when the coupled device is idle. As can be seen from the abovedescription, the OFF time of the first switch M2 will be prolonged whenthe isolated switching mode power supply 20 has no load or light load.As a result, the idle time of the coupled device is prolonged too. Thus,the power consumption of the coupled device and the auxiliary circuits,i.e., the resistor R2, could be reduced and the efficiency of the powersupply 20 is improved.

FIG. 4 schematically shows a primary controller 204 in accordance withan embodiment of the present invention. Compared with the primarycontroller 201 in FIG. 2, the primary controller 204 further has asecond feedback terminal FB2 coupled to the first terminal of the thirdwinding Lt to receive a second feedback signal Vfb2. The primarycontroller 204 further comprises: a load detecting circuit 401 having afirst input terminal configured to receive the second feedback signalVfb2, a second input terminal configured to receive the switching signalGate and an output terminal configured to provide a load detectingsignal 403 based on the second feedback signal Vfb2 and the switchingsignal Gate; a startup control circuit 402 having an input terminalconfigured to receive the current sense signal Vcs and an outputterminal configured to provide a startup control signal 404 based on thecurrent sense signal Vcs.

As can be seen from FIG. 2, the output voltage Vo is supplied to thepower terminal Vcc as the power supply of the secondary controller 202.Thus the frequency control signal Con and the frequency modulationsignal may be wrong during startup or during when the output voltage Vois too low. In that situation, the frequency control signal Con isblocked and the startup control circuit 402 instead of the logic circuit108 controls the primary power switch M1.

In one embodiment, the primary controller 204 further comprises aselector 118 having a first input terminal coupled to the outputterminal of the startup control circuit 402 to receive the startupcontrol signal 404, a second input terminal coupled to the outputterminal “Q” of the first RS flip-flop 106 to receive the logic controlsignal 111, a control terminal coupled to the output terminal of theload detecting circuit 401 to receive the load detecting signal 403 andan output terminal configured to provide the startup control signal 404or logic control signal 111 based on the load detecting signal 403.

In one embodiment, the selector 118 comprises a SPDT (Signal-PoleDouble-Throw) switch, wherein the SPDT switch has a first input terminalconfigured to receive the startup control signal 404, a second inputterminal configured to receive the logic control signal 111, a controlterminal configured to receive the load detecting signal 403 and anoutput terminal configured to provide the startup control signal 404 orthe logic control signal 111 base on the load detecting signal 403.

In one embodiment, the load detecting circuit 401 comprises: a loaddetecting comparator 121 having a first input terminal (inverting inputterminal) configured to receive the second feedback signal Vfb2, asecond input terminal (non-inverting input terminal) configured toreceive a second reference signal Vref2 and an output terminalconfigured to provide a load comparison signal 125 based on the secondfeedback signal Vfb2 and the second reference signal Vref2; a pulsegenerator 123 having an input terminal configured to receive theswitching signal Gate and an output terminal configured to generate apulse signal 124 based on the switching signal Gate; and a latch 126having a clock terminal coupled to the output terminal of the pulsegenerator 123 to receive the pulse signal 124, an input terminal coupledto the output terminal of the load detecting comparator 121 to receivethe load comparison signal 125 and an output terminal configured toprovide the load detecting signal 403 based on the pulse signal 124 andthe load comparison signal 125.

In one embodiment, the load detecting circuit 401 further comprises adelay circuit 120 having an input terminal configured to receive theswitching signal Gate and an output terminal configured to generate anenable signal EN to the output terminal of the pulse generator 123,wherein the delay circuit 120 delays the switching signal Gate so thatthe pulse generator 123 generates the pulse signal 124 some times laterafter the primary power switch M1 is turned OFF. The second feedbacksignal Vfb2 indicates the voltage across the third winding Lt which isproportional to the output voltage Vo when the secondary power switch D1is ON. So the second feedback signal Vfb2 is proportional to the outputvoltage Vo when the secondary power switch D1 is ON. In one embodiment,when the output voltage Vo is too low that the secondary controller 202could not operate properly, the second feedback signal Vfb2 is lowerthan the second reference signal Vref2, and the load detectingcomparator 121 flips. The load comparison signal 125 generated by theload detecting comparator 121 is latched by the latch 126 at the pulsegenerated by the pulse generator 123. Meanwhile, the latch 126 generatesthe load detecting signal 403 to indicate if the output voltage Vo istoo low. The selector 118 is controlled by the load detecting signal403. When the load detecting signal 403 indicates that the outputvoltage Vo is lower than the required value which could not be able toensure the proper work of the secondary controller 202, the startupcontrol signal 404 is selected to be the switching signal Gate tocontrol the primary power switch M1. Otherwise, the logic control signal111 is selected to be the switching signal Gate. The delay circuit 120is configured to filter the glitch of the second feedback signal Vfb2when the secondary power switch D1 is turned ON. Persons of ordinaryskill in the art should know that the value of the second referencesignal Vref2 and the delay time of the delay circuit 120 may bedifferent in different systems.

In one embodiment, the startup control circuit 402 comprises: a max-peakcurrent comparator 119 having a first input terminal configured toreceive the current sense signal Vcs, a second input terminal configuredto receive a max-peak current signal Vlim_max, and an output terminalconfigured to provide a max-peak current limit signal Vmp based on thecurrent sense signal Vcs and the max-peak current signal Vlim_max; anoscillator 114 configured to provide a clock signal Vosc; and a secondRS flip-flop 117 having a set terminal “S” coupled to the oscillator 114to receive the clock signal Vosc, a reset terminal “R” coupled to theoutput terminal of the max-peak current comparator 119 to receive themax-peak current limit signal Vmp and an output terminal “Q” configuredto provide the startup control signal 404 based on the clock signal Voscand the max-peak current limit signal Vmp.

In one embodiment, the clock signal Vosc generated by the oscillator 114has a constant frequency fs_max, which is also the maximum frequency ofthe isolated switching mode power supply 20.

FIG. 5 shows a flow chart 50 of a method of controlling an isolatedswitching mode power supply in accordance with an embodiment of thepresent invention. The isolated switching mode power supply comprises atransformer, a primary power switch and a secondary power switch,wherein the transformer has a primary winding, a secondary winding and athird winding, and wherein the primary power switch is coupled to theprimary winding and the secondary power switch is coupled to thesecondary winding. The method comprises: step 501, receiving an inputvoltage via the primary winding of the transformer; step 502, turning ONand OFF the primary power switch to transfer energy stored in theprimary winding to the secondary winding, and to provide an outputvoltage to the load; step 503, generating a first feedback signalindicative of the output voltage; step 504, generating a frequencymodulation signal based on the first feedback signal and a sawtoothsignal; step 505, generating a frequency control signal based on thefrequency modulation signal; step 506, generating a current sense signalindicative of a current flowing through the primary winding of thetransformer; step 507, generating a current limit signal based on thecurrent sense signal and a peak current signal; and step 508, generatinga switching signal to control the primary power switch based on thecurrent limit signal and the frequency control signal.

In one embodiment, the step 504 comprises: generating an error signalbased on the first feedback signal and a first reference signal;generating a first comparison signal based on the error signal and thesawtooth signal; and generating the frequency modulation signal based onthe first comparison signal.

In one embodiment, the frequency control signal has a similar waveformwith the first comparison signal. In one embodiment, the frequencycontrol signal may have a same phase with the first comparison signal.However, in other embodiments, the frequency control signal may have anreversed phase with the first comparison signal.

In one embodiment, the logical low time of the sawtooth signal is fixedto a constant value. That is to say, the sawtooth signal Vsaw increasesafter a constant time period t, and becomes logical low when it reachesthe error signal. And after the constant time period t, the sawtoothsignal Vsaw increases again. The operation repeats. The constant timeperiod t may be adjusted in difference systems.

In one embodiment, the sawtooth signal is synchronize with conduction ofthe secondary power switch. When the secondary power switch is turnedON, the sawtooth signal increases; when the secondary power switch isturned OFF, the modulation becomes logical low and keeps logical lowuntil the secondary power switch is turned ON in the immediately nextswitching cycle.

In one embodiment, the step 508 comprises: turning ON the primary powerswitch when the frequency control signal generates a pulse; and turningOFF the primary power switch when the current sense signal reaches apeak current signal.

FIG. 6 shows a flow chart 60 of a method of controlling an isolatedswitching mode power supply. The isolated switching mode power supplycomprises a transformer, a primary power switch and a secondary powerswitch, wherein the transformer has a primary winding, a secondarywinding and a third winding, and wherein the primary power switch iscoupled to the primary winding and the secondary power switch is coupledto the secondary winding. The method comprises: step 601, receiving aninput voltage via the primary winding of the transformer; step 602,turning ON and OFF the primary power switch to transfer energy stored inthe primary winding to the secondary winding, and to provide an outputvoltage to the load; step 603, generating a first feedback signalindicative of the output voltage; step 604, generating a frequencymodulation signal based on the first feedback signal and a sawtoothsignal; step 605, generating a frequency control signal based on thefrequency modulation signal; step 606, generating a current sense signalindicative of a current flowing through the primary winding of thetransformer; step 607, generating a current limit signal based on thecurrent sense signal and a peak current signal; step 608, generating alogic control signal to control the primary power switch based on thecurrent limit signal and the frequency control signal; step 609,generating a startup control signal based on the current sense signal;step 610, generating a load detecting signal based on a second feedbacksignal indicative of the output voltage of the isolated switching modepower supply, wherein the load detecting signal is valid when the secondfeedback signal is lower than a preset threshold; and step 611,selecting the startup control signal as a switching signal to controlthe primary power switch when the load detecting signal is valid andselecting the logic control signal as the switching signal to controlthe primary power switch when the load detecting signal is invalid.

In one embodiment, the step 604 further comprises: generating an errorsignal based on the first feedback signal and a first reference signal;generating a first comparison signal based on the error signal and thesawtooth signal; and generating the frequency modulation signal based onthe first comparison signal.

In one embodiment, the frequency control signal has the same phase withthe first comparison signal.

In one embodiment, the frequency control signal has the reversed phasewith the first comparison signal.

In one embodiment, the logical low time of the sawtooth signal is fixedto a constant value. That is to say, the sawtooth signal increases aftera fixed time period t, and becomes logical low when reaches the errorsignal. And after a fixed time period t, the sawtooth signal increasesagain. The operation repeats. The fixed time period t could be adjustedin difference system.

In one embodiment, the sawtooth signal is synchronize with conduction ofthe secondary power switch. When the secondary power switch is turnedON, the sawtooth signal increases; when the secondary power switch isturned OFF, the modulation becomes logical low and keeps logical lowuntil the secondary power switch is turned ON in the next switchingcycle.

In one embodiment, the step 608 comprises: turning ON the primary powerswitch at the pulse of the frequency control signal; turning OFF theprimary power switch when the current sense signal reaches a peakcurrent signal.

In one embodiment, the step 611 comprises: turning ON the primary powerswitch when the frequency control signal is logical high; and turningOFF the primary power switch when the current sense signal reaches apeak current signal.

An effective technique for controlling an isolated switching mode powersupply has been disclosed. While specific embodiments of the presentinvention have been provided, it is to be understood that theseembodiments are for illustration purposes and not limiting. Manyadditional embodiments will be apparent to persons of ordinary skill inthe art reading this invention.

We claim:
 1. An isolated switching mode power supply, comprising: aninput terminal configured to receive an input voltage; an outputterminal configured to provide an output voltage; a transformer having aprimary winding and a secondary winding respectively having a firstterminal and a second terminal, the first terminal of the primarywinding being coupled to the input terminal to receive the inputvoltage; a primary power switch having a first terminal coupled to thesecond terminal of the primary winding, a second terminal coupled to aprimary ground node and a control terminal; a secondary power switchcoupled between the first terminal of the secondary winding and theoutput terminal of the power supply; a secondary controller having apower terminal configured to receive the output voltage, a firstfeedback terminal configured to receive a first feedback signalindicative of the output voltage and a coupling control terminalconfigured to generate a frequency modulation signal based on the outputvoltage and the first feedback signal, wherein the frequency modulationsignal has pulses modulated based on the first feedback signal; acoupled device having an input side coupled between the output terminalof the power supply and the coupling control terminal to receive theoutput voltage and the frequency modulation signal, and an output sideconfigured to provide a frequency control signal based on the outputvoltage and the frequency modulation signal; and a primary controllerhaving a current sense terminal configured to receive a current sensesignal indicative of a current flowing through the primary winding, afrequency control terminal coupled to the output side of the coupleddevice to receive the frequency control signal, a second feedbackterminal configured to receive a second feedback signal indicative ofthe output voltage, and an output terminal configured to provide aswitching signal to the control terminal of the primary power switchbased on the current sense signal and the frequency control signal;wherein the primary controller comprises: a current limit comparatorhaving a first input terminal configured to receive the current sensesignal, a second input terminal configured to receive a peak currentsignal, and an output terminal configured to provide a current limitsignal based on the current sense signal and the peak current signal; alogic circuit having a first input terminal coupled to the output sideof the coupled device to receive the frequency control signal, a secondinput terminal coupled to the output terminal of the current limitcomparator to receive the current limit signal, and an output terminalconfigured to provide a logic control signal based on the frequencycontrol signal and the current limit signal; a load detecting circuithaving a first input terminal configured to receive the second feedbacksignal, a second input terminal configured to receive the switchingsignal and an output terminal configured to provide a load detectingsignal based on the second feedback signal and the switching signal; astartup control circuit having an input terminal configured to receivethe current sense signal and an output terminal configured to provide astartup control signal based on the current sense signal; and a selectorhaving a first input terminal coupled to the output terminal of thestartup control circuit to receive the startup control signal, a secondinput terminal coupled to the output terminal of the logic circuit toreceive the logic control signal, a control terminal coupled to theoutput terminal of the load detecting circuit to receive the loaddetecting signal and an output terminal configured to provide thestartup control signal or logic control signal based on the loaddetecting signal; wherein: a frequency of the switching signal decreasesas a load of the isolated switching mode power supply becomes light; andwherein the frequency control signal has a first logical state and asecond logical state, wherein a time period of the first logical stateof the frequency control signal is prolonged when the load of theisolated switching mode power supply changes from heavy to light, andthe second logical state of the frequency control signal evokes an “ON”operation of the primary power switch.
 2. The isolated switching modepower supply of claim 1, wherein the secondary controller comprises: anerror amplifier having a first input terminal configured to receive thefirst feedback signal indicative of the output voltage, a second inputterminal configured to receive a first reference signal, and an outputterminal configured to provide an error signal based on the firstfeedback signal and the first reference signal; an error comparatorhaving a first input terminal coupled to the output terminal of theerror amplifier to receive the error signal, a second input terminalconfigured to receive a sawtooth signal and an output terminalconfigured to provide a first comparison signal based on the errorsignal and the sawtooth signal; and a first switch having a firstterminal coupled to the coupling control terminal of the secondarycontroller, a second terminal coupled to a secondary ground node and acontrol terminal coupled to the output terminal of the error comparatorto receive the first comparison signal, wherein based on the firstcomparison signal, the first switch is turned ON and OFF to generate thefrequency modulation signal at the coupling control terminal.
 3. Theisolated switching mode power supply of claim 1, wherein the loaddetecting circuit comprises: a load detecting comparator having a firstinput terminal configured to receive the second feedback signal, asecond input terminal configured to receive a second reference signaland an output terminal configured to provide a load comparison signalbased on the second feedback signal and the second reference signal; apulse generator configured to receive the switching signal and togenerate a pulse signal based on the switching signal; and a latchhaving a clock terminal coupled to the pulse generator to receive thepulse signal, an input terminal coupled to the output terminal of theload detecting comparator to receive the load comparison signal, and anoutput terminal configured to provide the load detecting signal based onthe pulse signal and the load comparison signal.
 4. The isolatedswitching mode power supply of claim 1, wherein the startup controlcircuit comprises: a max-peak current comparator having a first inputterminal configured to receive the current sense signal, a second inputterminal configured to receive a max-peak current signal, and an outputterminal configured to provide a max-peak current limit signal based onthe current sense signal and the max-peak current signal; an oscillatorconfigured to provide a clock signal; and a second RS flip-flop having aset terminal coupled to the oscillator to receive the clock signal, areset terminal coupled to the output terminal of the max-peak currentcomparator to receive the max-peak current limit signal and an outputterminal configured to provide the startup control signal based on theclock signal and the max-peak current limit signal.
 5. A method ofcontrolling an isolated switching mode power supply, the isolatedswitching mode power supply comprising a transformer, a primary powerswitch and a secondary power switch, wherein the transformer has aprimary winding, and a secondary winding, and wherein the primary powerswitch is coupled to the primary winding and the secondary power switchis coupled to the secondary winding, the method comprising: receiving aninput voltage via the primary winding of the transformer; turning ON andOFF the primary power switch to transfer energy stored in the primarywinding to the secondary winding, and to provide an output voltage to aload; generating a first feedback signal indicative of the outputvoltage; generating a frequency modulation signal based on the firstfeedback signal and a sawtooth signal, wherein the frequency modulationsignal has pulses modulated based on the first feedback signal and thesawtooth signal; generating a frequency control signal based on thefrequency modulation signal by a coupled device; generating a currentsense signal indicative of a current flowing through the primary windingof the transformer; generating a current limit signal based on thecurrent sense signal and a peak current signal; and generating aswitching signal to control the primary power switch based on thecurrent limit signal and the frequency control signal, comprising:generating a logic control signal based on the frequency control signaland the current limit signal; generating a load detecting signal basedon a second feedback signal indicative of the output voltage and theswitching signal; generating a startup control signal based on thecurrent sense signal; and selecting to provide a startup control signalor the logic control signal as the switching signal based on the loaddetecting signal; wherein a frequency of the switching signal decreasesas the load of the isolated switching mode power supply becomes light;and wherein the frequency control signal has a first logical state and asecond logical state, wherein a time period of the first logical stateof the frequency control signal is prolonged when the load of theisolated switching mode power supply changes from heavy to light, andthe second logical state of the frequency control signal evokes an “ON”operation of the primary power switch.
 6. The method of claim 5, whereinthe step of generating the frequency modulation signal based on thefirst feedback signal and the sawtooth signal comprises: generating anerror signal based on the first feedback signal and a first referencesignal; generating a first comparison signal based on the error signaland the sawtooth signal; and generating the frequency modulation signalbased on the first comparison signal.
 7. The method of claim 6, whereinthe frequency control signal has the same phase with the firstcomparison signal.
 8. The method of claim 6, wherein the frequencycontrol signal has the reversed phase with the first comparison signal.9. The method of claim 5, wherein the step of generating the switchingsignal to control the primary power switch based on the current limitsignal and the frequency control signal comprises: turning ON theprimary power switch when the frequency control signal generates apulse; and turning OFF the primary power switch when the current sensesignal reaches a peak current signal.